The present invention is directed to a methods for electroplating a metallic coating, such as solder to an array of small, closely spaced electrical contacts or pads.
When an electronic component fails it often becomes economically necessary to remove the failed device rather than replace the entire assembly. In the case where the electronic component such as an integrated circuit has electrical leads which are solder bonded they can be easily removed by remelting the soldered connections and removing the defective components such as disclosed in U.S. Pat. No. 4,934,582. However, after removal of the defective integrated circuit and prior to bonding a replacement integrated circuit, it is necessary to replenish the solder on the substrate pads which are small and which are closely spaced together. For example, the pads are three of four mils wide and are spaced on less than 15 mil centers. In addition, there are other applications in microelectronic assemblies where it is desirable to add a coating such as solder to substrate pads after the substrate fabrication process is complete. For example, it is desirable to pre-tin pads on a substrate when normal rack plating of solder is not feasible.
In some cases, the substrate may already be partially populated with electronic components or have surface plating other than solder in some areas. In these cases a solder replenishment method which requires immersion in molten solder or heating of the board or assembly to solder melting temperatures may be unacceptable. In many cases, the geometry of the pads may be too small for addition of solder by means of wire soldering and soldering iron, or even dispensing of solder cream and subsequent reflow. And in other cases, prior to soldering, certain metallic coatings must be removed in order to make a satisfactory solder joint.
The present invention is directed to a method of selectively electroplating a metallic coating onto a plurality of closely spaced electrical contacts by providing a miniaturized electroplating cell which encompasses only the desired area containing the electrical contacts to be plated and provides a bussing structure to connect all of the electrical contacts needing electrical coating while protecting the rest of the assembly from unwanted contact with the electroplating chemicals.